Welding head and welding head assembly for an arc-welding system

ABSTRACT

The invention relates to an electric arc-welding welding head comprising a contact device and one or more wire feeder units, the contact device encompassing an electrode assembly, the electrode assembly comprising at least two fusible continuously-fed wire electrodes arranged in a contact device. An electrically insulated duct is provided for electric insulation of at least one of the electrodes so that the electrode is electrically insulated from other electrodes in the electrode assembly. The invention also relates to an electric arc-welding contact device and an electric arc-welding welding head assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. non-provisional applicationSer. No. 13/258,023 filed on Dec. 9, 2011, entitled “Welding Head AndWelding Head Assembly for an Arc-Welding System,” which is a nationalphase application, under 35 U.S.C. § 371, based on and claiming thebenefit of international patent application no. PCT/EP2009/053875, whichhas an international filing date of Apr. 1, 2009. The disclosures of theabove applications are each incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The invention relates to a welding head for an arc-welding system and awelding head assembly for an arc welding system, particularly forsubmerged arc welding comprising an electrode assembly with a multitudeof electrodes.

BACKGROUND OF THE INVENTION

Submerged arc welding is a fully mechanized welding method characterizedby high productivity and quality, often used for longer welding seems inthicker materials. During submerged arc welding one or more sequentiallyarranged welding electrodes melt in arcs.

The weld, particularly the melted material and the arc, are protectedbeneath a layer of pulverized flux. The flux melts in part during theprocess, thus creating a protecting layer of slag on the weld pool. Theelectrical current used in the process is relatively high, usuallywithin 300-1500 Ampere per electrode. The electrodes used in submergedarc welding are usually 2.5-6 mm in diameter.

Fluxes used in submerged arc welding are granular fusible mineralstypically containing oxides of manganese, silicon, titanium, aluminum,calcium, zirconium, magnesium and other compounds such as calciumfluoride. The flux is specially formulated to be compatible with a givenelectrode wire type so that the combination of flux and wire yieldsdesired mechanical properties. All fluxes react with the weld pool toproduce the weld metal chemical composition and mechanical properties.It is common practice to refer to fluxes as ‘active’ if they addmanganese and silicon to the weld, the amount of manganese and siliconadded is influenced by the arc voltage and the welding current level.

To find the highest productivity possible with submerged arc welding,with increased competitiveness as one result, one strives for increasedweld speed and the highest possible deposition rate, i.e. melted weldingconsumables, or really created joint material, per hour and electrode.

One solution to this known in the art is to use multiple electrodes,positioned sequentially in the direction of the weld seem. Usually 2-3electrodes are used, however, usage of up to 6 electrodes is known.

It is known that addition of metal which melts without an arc canimprove the deposition rate during welding. For instance, metal powderor metal wires can be added during arc welding.

GB 1400051 A discloses an arrangement to improve the deposition rateduring submerged arc welding of a workpiece 10. The general principle isillustrated in FIG. 1. Three continuously fed wire electrodes 30, 36, 40serially disposed along a weld seam are consumed in arcs while moving ina welding direction 20. The electrodes 30, 36, 40 are also known as“hot” or “hot wire” electrodes. Two additional electrodes 32, 38consumed without forming arcs are continuously fed into the molten weldpool 12 in the region of the arc of the middle electrode 36. Theseadditional electrodes 32, 38, also known as “cold wires” or “cold wireelectrodes” are in continuous short-circuit contact with the weld pool12. The two electrodes 32, 38 consumed in the molten weld pool 12without the formation of arcs are melted by resistance heating as wellas by the heat generated by the middle electrode 36. The process uses aflux to generate protective gases and slag, and to add alloying elementsto the weld pool 12. An additional shielding gas is not required. Priorto welding, a thin layer of flux powder is placed on the workpiecesurface. The arc moves along a joint line in welding direction 20 and asit does so, flux is fed to the welding area by a flux feed hopper. Asthe arc is completely covered by the flux layer, heat loss is extremelylow. This produces a thermal efficiency as high as 60%, compared with25% for manual metal arc. There is no visible arc light, welding isvirtually spatter-free and there is no need for fume extraction.

Even though the above mentioned solutions increase the deposition rateduring welding it is desirable to provide a better handling of such coldelectrodes.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electrode assembly whichallows for improved welding speed. Another object of the invention is toprovide a method with an improved welding speed in conjunction with asatisfying weld quality. Another object is to provide an apparatus forperforming such method.

The objects are achieved by the features of the independent claims. Theother claims and the description disclose advantageous embodiments ofthe invention.

According to a first aspect of the invention, an electric arc-weldingwelding head is proposed comprising a contact device and a feedingportion, the contact device encompassing an electrode assembly, theelectrode assembly comprising at least two fusible continuously-fed wireelectrodes arranged in a contact device. An electrically insulated ductis provided for electric insulation of at least one of the electrodes sothat the electrode is electrically insulated from other electrodes inthe electrode assembly in the contact device.

The electrically insulating duct comprises insulated portions in wirefeeder and wire straightening units as well as guiding tubes outside thefeeder and straightening units. The duct can be composed of a multitudeof electrically insulating sections along the electrically insulatedelectrode from a wire reservoir such as a wire bobbin to the contactdevice close to the workpiece to be welded. Preferably, the electricarc-welding head is employed for submerged arc welding. Submerged arcwelding can be operated as a fully-mechanized or automatic process,optionally it can be semi-automatic. Typical welding parameters arecurrent, arc voltage and travel speed which all can affect bead shape,depth of penetration and chemical composition of the deposited weldmetal from the electrodes.

Favorably, the welding head is compact and allows for an independentmanipulation of the at least one electrically insulated electrodecompared to the other electrodes in the electrode assembly of thewelding head. The electrodes can be supplied by rear mounted wirebobbins on a so called “column and boom” arrangement. A column and boomis a rail moving manipulator which carries the welding head. The columnand boom is used to position the welding head to the weld groove. Thecolumn and boom could also be designed to carry the welding equipmentsuch as power sources, flux handling equipment and electrode coils asknown in the art of arc-welding systems. Preferably, the electricallyinsulated electrode is a “cold wire” electrode which can be fed througha spring-loaded contact-device inside a ceramic tube. The one or morehot wire electrode and the cold wire electrode can have separate wirefeeders, making this a non-synergetic set-up, offering a much wider weldparameter window compared to a synergetic set-up.

Using a cold wire electrode in combination with a one or more hot wireelectrodes offers the possibility to deposit about the same volume aswith three hot wire electrodes with the additional benefit of a reducedheat generation, as the cold wire electrode only “melts” down into theweld pool and thus lowers the temperature. This in turn allows for adifferent heat impact on the weld workpiece which in many cases ishighly beneficial and opens up for higher productivity. The increase indeposition rate can be above 20%, with a prospect to an increase ofproductivity of almost 50% with optimized welding parameters. Theinvention allows for higher deposition rates without increasing thewelding speed. Expediently, the invention allows for retrofittingexisting welding systems, particularly for welding of large workpiecessuch as pipe-mills.

According to a favorable embodiment of the invention, the electricallyinsulated duct can comprise an electrically insulated wire conduit inthe contact device. The insulated wire conduit can be a ceramics tubearranged in the contact device. The ceramics tube can be inserted in thecontact device or a part of the contact device can be treated, e.g.oxidized, to form an oxide of sufficient thickness for electricalinsulation.

According to a favorable embodiment of the invention, the electricallyinsulated duct can comprise an electrically insulated portion in a wirestraightening unit for straightening one or more of the electrodes. Thehot and cold electrodes can be manipulated in the same wirestraightening device.

According to a favorable embodiment of the invention, an individual wirestraightening unit can be provided for the electrically insulatedelectrode separate from wire straightening units of other electrodes ofthe electrode assembly. Advantageously, the electrically insulatedelectrode can be manipulated independently from other, particularly hotwire electrodes.

According to a favorable embodiment of the invention, the electricallyinsulated duct can comprise an electrically insulated portion in a wirefeeder unit for feeding one or more electrodes towards a workpiece. Thehot and cold electrodes can be manipulated in the same wire feederdevice.

According to a favorable embodiment of the invention, an individual wirefeeder unit can be provided for the electrically insulated electrodeseparate from wire feeding units of other electrodes of the electrodeassembly. Advantageously, the electrically insulated electrode can bemanipulated independently from other, particularly hot wire electrodes.

According to a favorable embodiment of the invention, an individualspeed control unit can be provided for the electrically insulatedelectrode separate from speed control unit of other electrodes of theelectrode assembly. Advantageously, the electrically insulated electrodecan be manipulated independently from other, particularly hot wire,electrodes.

Favorably, the electrically insulated electrode can be fed independentlyfrom other electrodes in the contact device. This allows for acomfortable control of the welding process. The electrically insulatedelectrode, being a cold wire electrode, can be fed with a differentspeed, particularly with a variable speed and/or with a differentdiameter, independent from other electrodes in the contact device.

According to a favorable embodiment of the invention, the electrode inthe electrically insulated duct can be arranged in sequential orderbetween a leading and a trailing electrode with respect to a weldingdirection on a workpiece. The order of the electrically insulatedelectrode can be used to adjust the welding deposition rate. Such asymmetric arrangement can yield very high deposition rates. Thesequential order of the electrically insulated electrode can be chosendepending on welding requirements.

According to a favorable embodiment of the invention, the electrode inthe electrically insulated duct can be arranged upstream of the otherelectrodes of the electrode assembly with respect to a welding directionon a workpiece. Depending on welding requirements the sequential orderof the electrically insulated electrode can be chosen. A benefit of anasymmetric arrangement of the electrodes is that the cold wire can bemore easily controlled in case any kind of adjustment of an angle ofapproach is required. The outermost electrodes can be tilted to adesired angle compared to the other electrodes. An asymmetricarrangement particularly allows adjusting such an angle in astraightforward manner.

According to a favorable embodiment of the invention, the electrode inthe electrically insulated duct can be arranged downstream of the otherelectrodes of the electrode assembly with respect to a welding directionon a workpiece. The sequential order of the electrically insulatedelectrode can be chosen depending on welding requirements.

According to a favorable embodiment of the invention, a feeder unitand/or a straightening unit for the electrodes, other than theelectrically insulated electrode, can provide a feedthrough for guidingthe electrically insulated electrode through the feeder unit. Favorably,the electrically insulated electrode can pass in the feedthrough throughthe wire straightening unit or the wire feeder unit without interactingwith these units or the other electrodes. The electrically insulatedelectrode can be manipulated independently from the other electrodes inthe same contact device of the same welding head.

According to another aspect of the invention, a welding head assembly isproposed comprising at least two electric arc-welding welding heads, atleast one of which comprises a contact device and a feeding portion, thecontact device encompassing an electrode assembly, wherein the electrodeassembly comprises at least two fusible continuously-fed wire electrodesarranged in a contact device. An electrically insulated duct is providedfor electric insulation of at least one of the electrodes so that theelectrode is electrically insulated from other electrodes of theelectrode assembly. Favorably, such a welding head assembly is apowerful tool for very high deposition rates with reduced heat impact onworkpieces. The welding quality can thus be improved.

According to a favorable embodiment of the invention, a first weldinghead can be arranged in sequential order before a second welding head sothat the electrode assemblies of each welding head are aligned along awelding direction during a welding operation in sequential order. Thewelding head according to the invention allows for a large degree offreedom when designing such a welding head assembly.

According to a favorable embodiment of the invention, an order of theelectrically insulated electrode in each electrode assembly can be thesame for each welding head with respect to the other electrodes in eachelectrode assembly. The welding head according to the invention allowsfor a large degree of freedom when designing such a welding headassembly.

According to a favorable embodiment of the invention, an order of theelectrically insulated electrode in one electrode assembly can bereversed with respect to an order of the electrically insulatedelectrode in another electrode assembly of the welding head arrangement.The welding head according to the invention allows for a large degree offreedom when designing such a welding head assembly.

According to a favorable embodiment of the invention, each electricallyinsulated electrode can be provided with a wire straightening unitand/or a wire feeder unit separate from wire straightening units and/orfeeder units of the other electrodes of the electrode assemblies. Theelectrically insulated electrodes can be manipulated independently fromthe other electrodes and independently in each welding head thus givinga large degree of freedom in adapting welding parameters for particularwelding requirements imposed by material and/or geometry and/or ambientconditions of the workpieces to be welded.

According to another aspect of the invention, an electric arc-weldingcontact device for an electric arc-welding head and/or a welding headassembly is proposed, comprising a device body and an electricallyinsulated portion for guiding an electrode in an electrically insulatedway through the contact device and at least one duct for at leastanother electrode which is in electrical contact with the device body.Favorably, a compact device is provided where one or more electricallyinsulated electrodes can be employed together with one or morenon-insulated electrodes. The one or more electrically non-insulatedelectrodes can be provided for being consumed in arcs during weldingwhereas the one or more electrically insulated electrodes can be socalled cold electrodes which are not consumed by arcs but are intendedto melt in the weld pool during welding. The electrically insulated wirecan be used with various diameters chosen independently from the otherelectrodes in the contact device.

According to a favorable embodiment of the invention, the electricallyinsulated portion can comprise an electrically insulating tube. Anelectrically insulating tube can be easily inserted into the devicebody. Expediently, the tube can be made of ceramics which provides asuperior electrical insulation as well as a good heat resistance.

According to a favorable embodiment of the invention, the device bodycan be composed of a main part and a removable part which can beattached to the main part. An arrangement of the electrodes in a desiredsequential order can be easily achieved or changed.

According to a favorable embodiment of the invention, a tip can providedat one end of the device body which has recesses for receiving theelectrically insulating portion for an electrically insulating electrodeand ducts for other electrodes. Favorably, the one or more electrodesand the electrically insulating portion can be arranged in the groovesof the tip in a defined way and held securely during operation.

According to a favorable embodiment of the invention, the electricallyinsulating portion can be arranged in a symmetric position between ductsfor other electrodes. Favorably, the electrically insulated portion forguiding an electrode can be arranged between two ducts of electrodes.Such a symmetric arrangement provides a high deposition rate duringwelding

According to a favorable embodiment of the invention, the electricallyinsulating portion can be arranged in an eccentric position in relationto ducts for other electrodes in the device body. Particularly, theelectrically insulating portion can be provided during welding under adesired angle when approaching the weld pool by tilting the contactdevice.

According to another aspect of the invention, an electric arc-weldingsystem is provided for generating a weld pool in a workpiece, comprisingat least one welding head or at least one welding head assemblyaccording to anyone of the features described above. A highly versatilesystem can be provided which allows for easy adjustment of the weldingparameters and conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above-mentioned and otherobjects and advantages may best be understood from the followingdetailed description of the embodiments, but not restricted to theembodiments, wherein is shown schematically:

FIG. 1: a welding arrangement with hot and cold electrodes according tothe prior art;

FIG. 2: a first view of an example embodiment of an arc-welding weldinghead according to the invention for submerged arc welding;

FIG. 3: the arc-welding welding head of FIG. 1 turned counterclockwiseby 90°;

FIG. 4: a perspective view or the arc-welding head of FIG. 1;

FIG. 5a-5e : various views of a contact device for three wire electrodeswith cut open views (FIGS. 5a, 5e ) and side views (FIGS. 5b, 5d ) and afront view (FIG. 5c );

FIG. 6: a side view of an example embodiment of an arc-welding weldinghead

FIG. 7a-7c : an arrangement of an electrically insulated electrode and ahot electrode in a first electrode assembly comprising two electrodes(FIG. 7a ), a second electrode assembly comprising three electrodes withthe electrically insulated electrode being the central electrode (FIG.7b ), and a third electrode assembly comprising three electrodes withthe electrically insulated electrode being at the outside of theelectrode assembly (FIG. 7c ); and

FIG. 8a-8c : an arrangement of two arc-welding welding heads eachcomprising an electrically insulated electrode and a hot electrode in afirst electrode assembly comprising two electrodes in each welding head(FIG. 8a ), a second electrode assembly comprising three electrodes ineach welding head, with the electrically insulated electrode being thecentral electrode (FIG. 8b ), and a third electrode assembly comprisingthree electrodes in each welding head, with the electrically insulatedelectrode being at the outside of the electrode assembly (FIG. 8c ).

DETAILED DESCRIPTION

In the drawings, equal or similar elements are referred to by equalreference numerals. The drawings are merely schematic representations,not intended to portray specific parameters of the invention. Moreover,the drawings are intended to depict only typical embodiments of theinvention and therefore should not be considered as limiting the scopeof the invention.

FIGS. 2 to 4 depict different views of an example embodiment of anelectric arc-welding welding head 100 according to the invention forsubmerged arc welding which views are described in combination.

Along its longitudinal extension the electric arc-welding welding head100 comprises a contact device 160 at its lower end which during weldingis in close proximity to the workpiece to be welded. The contact device160 holds the electrodes 172, 174, 176 (FIGS. 3, 4) of the electrodeassembly 170 of the welding head 100. The electrodes 172, 174, 176 exitthe contact device 160 through an outlet 162 at the lower end of thecontact device 160 which faces the workpiece during the weldingoperation. The wire electrodes 172, 174, 176 may be fed from respectivereservoirs such as coils (not shown) towards the arc welding head 100.

The electrode assembly 170 comprises by way of example three fusiblecontinuously-fed wire electrodes 172, 174, 176 arranged in the contactdevice 160. The contact device 160 comprises an electrically insulatingportion 166 for an electrically insulated wire electrode 174. Only theelectrically insulated electrode 174 is shown along the welding headextension. The other electrodes 172, 176 are only shown in the contactdevice 160. The non-insulated electrodes 172, 176 can be arranged as socalled twin wires which are fed in parallel as a double wire electrodearrangement. The electrically insulating portion 166 is preferably anextra insulated wire conduit, for instance made of heat resistantmaterial such as ceramics.

Above the contact device 160 a feeder device 150 is arranged which feedsthe electrodes towards the contact device 160. Typically, the feederunit 150 comprises grooved wheels which move the wire electrodes 172,176 (not shown in this portion for clarity reasons) towards the contactdevice 160. The feeder unit 150 comprises an electrically insulatingportion 156 for feeding through the electrically insulated electrode174. The electrically insulating portion 156 can consist of feederwheels with an extra insulated groove for the electrically insulatedelectrode 174. The electrically insulated electrode 174 can pass throughthe wire feeder unit 150 freely. The feeder wheels are driven by adriving unit 152, e.g. an electric motor.

Aside from the wire feeder unit 150 a flux hopper 114 is arranged whichfeeds granular flux to the contact device 160 via a nozzle (not shown)for submerged arc welding. Besides the driving unit 152 the wire feederunit 150 comprises a gear with a drive shaft. On the drive shaft of thegear a feeding wheel 154 (FIG. 6) is arranged which can be pressurizedby another wheel (not shown). The feeding wheel 154 drives the wireelectrode forward in the direction of the contact device 160.

Above the wire feeder unit 150 a wire straightening unit 140 is arrangedfor straightening the wire electrodes 172, 176. Two rollers (notreferred to with reference numbers) depicted in a foremost position ofthe wire straightening unit 140 are used to exert a pressure on threefixed wheels arranged vertically one over the other in the rear part ofthe wire straightening device. The pressure the rollers are exerting onthe wheels is adjustable via knobs at the outside of the wirestraightening unit 140. The pressure of the rollers on the three wheelsis straightening the wire. The wire straightening unit 140 comprises anelectrically insulating portion 146 through which the electricallyinsulated electrode 174 can pass freely through the wire straighteningunit 140.

Above the wire straightening unit 140 a separate wire feeder unit 130 isdisposed for feeding the electrically insulated electrode 174 towardsthe contact device 160. On the wire feeder unit 130 a driving unit 132,e.g. an electric motor, is arranged which drives feeder wheels of thewire feeder unit 130. Besides the driving unit 132, the wire feeder unit130 comprises a gear with a drive shaft. On the drive shaft of the geara feeding wheel 134 (FIG. 6) is arranged which can be pressurized byanother wheel (not shown). The feeding wheel 134 drives the wireelectrode forward in the direction of the contact device 160.

Above the wire feeder unit 130 a separate wire straightening unit 120 isarranged for straightening the electrically insulated electrode 174.Thus, along the longitudinal extension of the welding head 100 anelectrically insulating duct 180 is provided for guiding theelectrically insulated electrode 174 from a wire reservoir such as awire bobbin (not shown) to the contact nozzle. Between the feeder units150 and 130 and above the wire straightening unit 120 an electricallyinsulated wire conduit can be arranged which receives the electricallyinsulated electrode 174.

Particularly, the electrically insulating duct 180 consists of theelectrically insulating portion 146 of the wire straightening unit 140,the electrically insulating portion 156 of the wire feeder unit 150 forthe non-insulated electrodes 172, 176, and the electrically insulatedportion 166 of the contact device 160 as well as electrically insulatedwire conduits between and the units 130, 140, 150, 160 and above thewire straightening unit 120 for the electrically insulated electrode174.

FIG. 5a to FIG. 5e depict various views of a contact device 160 whichare described in combination. The contact device 160 encompasses adevice body 161 which comprises ducts for receiving wire electrodes (notshown), wherein an electrically insulated portion 166 is one of theducts.

By way of example the device body 161 is made of two parts 161 a, 161 bseparated in longitudinal direction where a removable part 161 b isattached to the main part 161 a. The main part 161 a has a cylindricalupper portion and a lower portion which is slightly recessed in relationto the cut through the middle (FIG. 5a, 5e ). The removable part 161 bis attached to the recessed portion of the main part 161 a by screws.

Alternatively, the device body 161 can be a monolithic metal body withone or more bores drilled through for the passage of the wire electrodesinstead of several pieces. Between the removable part 161 b and the mainpart 161 a remains a small free distance when the parts 161 a and 161 bare attached to one another. The distance allows for using various wirediameters for the electrodes in the contact device 160. As theelectrically insulated electrode is guided separately in theelectrically insulated portion 166 the diameter of the electricallyinsulated electrode can be chosen independently from the diameters ofthe other electrodes in the device body 161.

The parts 161 a, 161 b of the device body 161 enclose a tip 163 which iscomposed of two parts 163 a, 163 b wherein part 163 a is attached to themain part 161 a and part 163 b is attached to the removable part 161 bof the device body 161. The parts 163 a, 163 b of the tip 163 can beattached to the main part 161 a and the removable part 161 b,respectively, by screws inserted in respective borings 164 a, 164 bprovided in the main part 161 a and the removable part 161 b.

The tip 163 may have a smaller diameter than the rest of the device body161 which may have predominantly a cylindrical shape. The electricallyinsulating portion 166 extends throughout the longitudinal extension ofthe contact device 166 and provides an outlet 169 for the electricallyinsulated wire electrode (not shown) at an outlet 162 of the tip 163 ofthe contact device 160.

As can be seen in the cut open views presented in FIGS. 5a and 5e , thedevice body 161 has an interior section 167 with the electricallyinsulating portion 166 arranged in a central position, providing anelectrically insulating portion for an electrically insulated wireelectrode (not shown). The electrically insulating portion 166 ispreferably an insulating tube such as a ceramics tube. On both sides ofthe central electrically insulating portion 166 a duct 168 a, 168 b isprovided for wire electrodes (not shown) which are intended to beconsumed by arcs during welding. The ducts 168 a, 168 b comprisecorresponding groove portions in the tip parts 163 a, 163 b. The grooveportions form an enclosure for the wire electrodes when the removablepart 161 b is attached to the main part 161 a of the device body 161.

At the outside of the device body 161 a bolt 165 is arranged which holdsthe parts 161 a, 161 b of the device body together by way of springelements such as e.g. disk springs, providing a contact pressure of theparts 161 a, 161 b virtually independent of the actual diameter of thewire electrodes.

An electrical contact can be attached to the contact device 160 incontact portion 164 c arranged in the main part 161 a of the device body161. A wire connected to a power source can be connected to the contactportion 164 c for transferring electric voltage and current to thecontact device 160 so that arcs can develop at the non-insulatedelectrodes which are in close electric contact to the device body 161.

In the electrically insulating portion 166 the wire electrode can passthrough the contact device 160 without contacting the other electrodes(not shown).

FIG. 6 is a side view of an example embodiment of an arc-welding weldinghead 100 of virtually the same layout as shown in FIGS. 2 to 4. In orderto avoid unnecessary repetitions it is referred to these drawings for adetailed description of like elements. Above the wire straightening unit140 two guide tubes 142, 144 are provided for twin electrodes. The guidetubes 142, 144 are arranged crosswise to the longitudinal extension ofthe welding head 100. Between the wire feeder unit 130 for theelectrically insulated electrode (not shown) and the wire straighteningunit 140 of the non-insulated electrodes (not shown) a guide tube 182for the electrically insulated electrode (not shown) is arranged. Thedriving units 132, 152 can be equipped with pulse sensors for speedcontrol of the electrodes. Close to the contact device 160 a nozzle 116for a flux hopper 114 (FIGS. 2-4) is arranged, the nozzle 116 is fixedto a rod 116 arranged parallel to the longitudinal axis of the contactdevice 160.

FIGS. 7a to 7c display arrangements of electrodes in electrodeassemblies 170 with respect to a welding direction 20.

FIG. 7a shows a first variant of an electrode assembly 170 with anelectrically insulated “cold” electrode 174 and a non-insulated “hot”electrode. A second variant of an electrode assembly 170 is shown inFIG. 7b comprising three electrodes 172, 174, 176 with the electricallyinsulated electrode 174 being the central electrode. A third variant ofan electrode assembly 170 comprising three electrodes 172, 174, 176 withthe electrically insulated electrode 174 being at the outside of theelectrode assembly is displayed in FIG. 7 c.

FIGS. 8a to 8c show arrangements of a welding head assembly 200comprising two arc-welding welding heads 100 a, 100 b. Each welding head100 a, 100 b comprises an electrode assembly 170 a, 170 b with anelectrically insulated electrode 174 a, 174 b and one or more hotelectrodes 172 a, 172 b. In a first variant of a welding head assembly200 shown in FIG. 8a each welding head 100 a, 100 b comprises twoelectrodes 172 a, 174 a, 172 b, 174 b in each welding head 100 a, 100 b.The electrically insulated electrodes 174 a, 174 b are the outermostelectrodes in the welding head assembly 200.

A second variant is shown in FIG. 8b , where each welding head 100 a,100 b comprises an electrode assembly 170 a, 170 b with three electrodes172 a, 174 a, 176 a, 172 b, 174 b, 176 b, wherein the electricallyinsulated electrode 174 a, 174 b is arranged in the middle between thetwo non-insulated electrodes 172 a, 176 a, 172 b, 176 b as centralelectrodes of each welding head 100 a, 100 b.

FIG. 8c shows a third variant with three electrodes 172 a, 174 a, 176 a,172 b, 174 b, 176 b in each welding head 100 a, 100 b, with theelectrically insulated electrode 174 a, 174 b being at the outside ofthe welding head assembly 200.

1. A welding head assembly comprising a first electric arc-weldingwelding head and a second electric arc-welding welding head, wherein thefirst welding head comprises: at least one fusible, continuously-fed hotwire electrode; a fusible, continuously-fed cold wire electrode; and acontact device with an electrically insulated duct and at least oneadditional duct, wherein the at least one hot wire electrode extendsthrough the at least one additional duct and the cold wire electrodeextends through the electrically insulated duct so that the cold wireelectrode is electrically insulated from the at least one hot wireelectrode.
 2. The welding head assembly of claim 1, wherein, during awelding operation, the at least one hot wire electrode is melted by anarc and the cold wire electrode is melted by one or more of: a weld poolgenerated by melting the at least one hot wire electrode; and heatgenerated when melting the at least one hot wire electrode.
 3. Thewelding head assembly of claim 2, wherein the weld pool melts the coldwire electrode via resistance heating.
 4. The welding head assembly ofclaim 1, wherein the first welding head and the second welding head areboth aligned with respect to a welding direction during a weldingoperation.
 5. The welding head assembly of claim 1, wherein the firstwelding head is ahead of the second welding head with respect to thewelding direction.
 6. The welding head assembly of claim 1, wherein thefirst welding head is behind the second welding head with respect to thewelding direction.
 7. The welding head assembly of claim 1, wherein theat least one hot wire electrode comprises at least one first hot wireelectrode, the cold wire electrode comprises a first cold wireelectrode, the contact device comprises a first contact device with afirst electrically insulated duct and at least one first additionalduct, and the second welding head comprises: at least one fusible,continuously-fed second hot wire electrode; a fusible, continuously-fedsecond cold wire electrode; and a second contact device with a secondelectrically insulated duct and at least one second additional duct,wherein the at least one second hot wire electrode extends through theat least one second additional duct and the second cold wire electrodeextends through the second electrically insulated duct so that thesecond cold wire electrode is electrically insulated from the at leastone second hot wire electrode.
 8. The welding head assembly of claim 7,wherein the at least one first hot wire electrode and the first coldwire electrode are ordered in a first order and the at least one secondhot wire electrode and the second cold wire electrode are ordered in asecond order.
 9. The welding head assembly of claim 8, wherein the firstorder is identical to the second order.
 10. The welding head assembly ofclaim 8, wherein the first order is opposite to the second order. 11.The welding head assembly of claim 8, wherein the first order and thesecond order collectively position the first cold wire electrode and thesecond cold wire electrode outside of both the at least one first hotwire electrode and the at least one second hot wire electrode.
 12. Thewelding head assembly of claim 1, wherein the first welding head furthercomprises: a wire straightening unit for straightening the at least onehot wire electrode, the wire straightening unit including anelectrically insulating portion for the cold wire electrode thatelectrically insulates the cold wire electrode from the at least one hotwire electrode while the wire straightening unit straightens the atleast one hot wire electrode.
 13. The welding head assembly of claim 1,wherein the first welding head further comprises: a wire feeder unit forfeeding the at least one hot wire electrode towards the contact device,the wire feeder unit including an electrically insulating portion forthe cold wire electrode that electrically insulates the cold wireelectrode from the at least one hot wire electrode while the wire feederunit feeds the at least one hot wire electrode towards the contactdevice.
 14. The welding head assembly of claim 1, wherein the contactdevice includes a tip and the electrically insulated duct extendslongitudinally through the contact device to provide an outlet for thecold wire electrode at an opening provided by the tip of the contactdevice.
 15. The welding head assembly of claim 14, wherein the outlet isdisposed in a central position within the opening of the tip.
 16. Thewelding head assembly of claim 14, wherein the outlet is disposed in aneccentric position within the opening of the tip.
 17. The welding headassembly of claim 1, wherein the electrically insulated duct comprisesan electrically insulating tube.
 18. A welding head assembly comprising:a first electric arc-welding welding head comprising: a first fusible,continuously-fed hot wire electrode; a fusible, continuously-fed coldwire electrode; and a first contact device with a first electricallyinsulated duct and a first additional duct, wherein the first hot wireelectrode extends through the first additional duct and the first coldwire electrode extends through the first electrically insulated duct sothat the first cold wire electrode is electrically insulated from thefirst hot wire electrode; and a second electric arc-welding welding headcomprising: a second fusible, continuously-fed hot wire electrode; asecond, continuously-fed cold wire electrode; and a second contactdevice with a second electrically insulated duct and a second additionalduct, wherein the second hot wire electrode extends through the secondadditional duct and the second cold wire electrode extends through thesecond electrically insulated duct so that the second cold wireelectrode is electrically insulated from the second hot wire electrode.19. The welding head assembly of claim 18, wherein the first cold wireelectrode is also electrically insulated from the second hot wireelectrode, and wherein the second cold wire electrode is alsoelectrically insulated from the first hot wire electrode.
 20. Thewelding head assembly of claim 18, wherein the first welding head andthe second welding head are arranged so that the first cold wireelectrode, the first hot wire electrode, the second cold wire electrode,and the second hot wire electrode are aligned with respect to a weldingdirection.